Polyelectrolyte complexes are substances which are ionically crosslinked hydrogels formed by the coreaction of highly and oppositely charged electrolytes. While polyelectrolytes can be formed from natural polymers, such as gums, gelatin and the like, interest has centered on polyelectrolyte complexes of synthetic polymers. Of greatest commercial interest is a polyelectrolyte complex formed between sodium polystyrene sulfonate and polyvinylbenzyltrimethylammonium chloride. These materials are characterized as a strongly acidic polyanion (sodium polystyrene sulfonate) and a strongly basic polycation (polyvinylbenzyltrimethylammonium chloride). While it has been stated that weak acid--weak base polyelectrolytes can interact to form what may be polyelectrolyte complexes, it is also stated that,
"However, the resulting hydrogels are not as strong mechanically, their degree of hydration is more difficult to control, and they are thermally and chemically less stable than PEC hydrogels prepared from polymeric salts of strong acids and strong bases." (Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 16, 1977 p. 118).
While attention, therefore, has centered on the above described strong acid-strong base type of complex, these materials have been found to be difficult to handle. U.S. Pat. No. 3,276,598 is directed to a method for forming this film by the reaction at the interface of aqueous solutions of sodium polystyrene sulfonate and polyvinylbenzyltrimethylammonium chloride. The thus-formed film blocks further interaction rendering the process self-limiting with the film thickness only about 200 A.
To produce solutions of such polyelectrolyte complexes so that films or membranes can be cast therefrom require ternary solvent compositions of inorganic electrolyte, polar organic solvent and water, such as calcium nitrate decahydrate, dioxane and water. See for example, U.S. Pat. Nos. 3,467,604 and 3,271,496. Employing such solvent systems would result in processing problems due to incompatibilities with other systems. For example, the aforementioned solvent systems preclude film coating by casting and drying since the non-volatile solvent components cannot be removed without further processing.
Chitin and chitosan films and membranes are known to the art, both as the sole component and in combination with another material; for example, membranes of chitosan and polyvinyl alcohol, chitosan and cellulose acetate and sulfoethyl chitosan. Chitin membranes are generally prepared by acetylating chitosan membranes. See for example, R. A. A. Muzzarelli, Chitin, Pergamon Press, 1977.
Japanese Application No. 155568/68, application date Dec. 16, 1978 (Laid open No. 81705/80, Laid open date June 20, 1980) is directed to a method for manufacturing chitosan film by formig an acidic chitosan solution, adding propylene glycol to the solution, forming a film, treating with alkali and then removing the polyethylene glycol with water.
Kikuchi and Fukuda, in Die Makromol. Chem., 175, 3573 (1974) describe a polyelectrolyte complex of sodium dextran sulfate and chitosan. However, it is stated that there is no solvent available for such a complex and, therefore films cannot be prepared.
Fukuda, in Bull. Chem. Soc Jap., b 53, 837 (1980) describes a polyelectrolyte complex of chitosan and sodium carboxymethyl cellulose. This complex is only soluble at a pH of less than 2 and only glacial formic acid is disclosed as a solvent.
U.S. Pat. No. 4,301,067, issued Nov. 17, 1981, is directed to a poly-ion complex comprising chitin or N-acylchitosan derivative having carboxymethyl and polyelectrolyte. The N-acyl derivative of chitin or chitosan is necessary to obtaining solubility. In this manner high molecular polyelectrolyte can be employed to form the poly-ion complex. The poly-ion complex is described as soluble in aqueous salt solutions and in such a dissolved state is shaped into film-like or filter-like bodies and then insolubilization is carried out by desalting treatment.